Geology topics

M. Vergnolle

Publications and source records attributed to M. Vergnolle.

3 recordsLinked to original sources

USGS2006

Mechanical deformation model of the western United States instantaneous strain-rate field

We present a relationship between the long-term fault slip rates and instantaneous velocities as measured by Global Positioning System (GPS) or other geodetic measurements over a short time span. The main elements are the secularly increasing forces imposed by the bounding Pacific and Juan de Fuca (JdF) plates on the North American plate, viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods, and steady slip along creeping portions of faults in the context of a thin-plate system. In detail, the physical model allows separate treatments of faults with known geometry and slip history, faults with incomplete characterization (i.e. fault geometry but not necessarily slip history is available), creeping faults, and dislocation sources distributed between the faults. We model the western United States strain-rate field, derived from 746 GPS velocity vectors, in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding Pacific and JdF plates. Relaxation following major earthquakes (M ??? 8.0) strongly shapes the present strain-rate field over most of the plate boundary zone. Equally important are lateral shear transmitted across the Pacific-North America plate boundary along ???1000 km of the continental shelf, downdip forces distributed along the Cascadia subduction interface, and distributed slip in the lower lithosphere. Post-earthquake relaxation and tectonic forcing, combined with distributed deep slip, constructively interfere near the western margin of the plate boundary zone, producing locally large strain accumulation along the San Andreas fault (SAF) system. However, they destructively interfere further into the plate interior, resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone. Much of the right-lateral strain accumulation along the SAF system is systematically underpredicted by models which account only for relaxation from known large earthquakes. This strongly suggests that in addition to viscoelastic-cycle effects, steady deep slip in the lower lithosphere is needed to explain the observed strain-rate field. ?? 2006 The Authors Journal compilation ?? 2006 RAS.

Geophysical Journal International↗

USGS2003

Constraints on the viscosity of the continental crust and mantle from GPS measurements and postseismic deformation models in western Mongolia

We use GPS measurements and models of postseismic deformation caused by seven M6.8 to 8.4 earthquakes that occurred in the past 100 years in Mongolia to assess the viscosity of the lower crust and upper mantle. We find an upper mantle viscosity between 1 ?? 1018 and 4 ?? 1018 Pa s. The presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath Mongolia. The viscosity of the lower crust is less well constrained, but a weak lower crust (3 ?? 1016 to 2 ?? 1017 Pa s) is preferred by the data. Using our best fit upper mantle and lower crust viscosities, we find that the postseismic effects of viscoelastic relaxation on present-day horizontal GPS velocities are small (<2 mm yr-1) but still persist 100 years after the 1905, M8.4, Bolnay earthquake. This study shows that the GPS velocity field in the Baikal-Mongolia area can be modeled as the sum of (1) a rigid translation and rotation of the whole network, (2) a 3-5 mm yr-1 simple shear velocity gradient between the Siberian platform to the north and northern China to the south, and (3) the contribution of postseismic deformation, mostly caused by the 1905 Bolnay-Tsetserleg sequence and by the smaller, but more recent, 1957 Bogd earthquake. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2003

Fault interaction and stress triggering of twentieth century earthquakes in Mongolia

A cluster of exceptionally large earthquakes in the interior of Asia occurred from 1905 to 1967: the 1905 M7.9 Tsetserleg and M8.4 Bolnai earthquakes, the 1931 M8.0 Fu Yun earthquake, the 1957 M8.1 Gobi-Altai earthquake, and the 1967 M7.1 Mogod earthquake (sequence). Each of the larger (M ??? 8) earthquakes involved strike-slip faulting averaging more than 5 m and rupture lengths of several hundred kilometers. Available geologic data indicate that recurrence intervals on the major source faults are several thousands of years and distances of about 400 km separate the respective rupture areas. We propose that the occurrences of these and many smaller earthquakes are related and controlled to a large extent by stress changes generated by the compounded static deformation of the preceding earthquakes and subsequent viscoelastic relaxation of the lower crust and upper mantle beneath Mongolia. We employ a spherically layered viscoelastic model constrained by the 1994-2002 GPS velocity field in western Mongolia [Vergnolle et al., 2003]. Using the succession of twentieth century earthquakes as sources of deformation, we then analyze the time-dependent change in Coulomb failure stress (????f). At remote interaction distances, static ????f values are small. However, modeled postseismic stress changes typically accumulate to several tenths of a bar over time intervals of decades. Almost all significant twentieth century regional earthquakes (M ??? 6) with well-constrained fault geometry lie in positive ????f lobes of magnitude about +0.5 bar. Our results suggest that significant stress transfer is possible among continental faults separated by hundreds of kilometers and on timescales of decades. Copyright 2003 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗